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82C55

ProgrammablePeripheralInterface

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Protocols

3 methods to transfer data from one device toother exists in micro computers.

1. Programmed I/O or Basic I/O

When data transferred between two devices is based on theexecution of data transfer program, without exchanging

any handshake signals before or after data transfer.2. Hand shaking I/O

Before transferring the data between two devices , few handshaking signals are exchanged between the two entitiesto ensure the readiness of device for the upcoming data& few hand shaking signals are exchanged after the data

transfer to signal the receiving side that data transfer isfinished.

3. Direct memory access (DMA)

Already studied.


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About 82C55

It is used to interface 8 bit parallel I/Odevice to a microprocessor.

It is used to interface to the keyboardand a parallel printer port in PCs.

PPI has 24 pins for I/O that areprogrammable in groups of 12 pinsand has three distinct modes ofoperation.


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82C55 : Pin Layout

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Control register for mode

Programming 82C55


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Mode 0

Port A Upper C Port BLower C

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Mode 0 (Basic Input/Output).

This mode provides simple input and

output operations for each of the three

ports.

No handshaking is required, data is

simply written to or read from a specified

port.


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82C55: Mode 0, 7 segment multiple

digit Display


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82C55: Mode 0, 7 segment multiple

digit Display

Port A provides the segment data inputs to

display and port B provides a means of

selecting one display position at a time.

Different values are displayed in each digit

via fast time division multiplexing.

http://www.google.com.pk/imgres?imgurl=http://www.johnloomis.org/altera/DE2/segments.jpg&imgrefurl=http://www.johnloomis.org/altera/DE2/seven_segment.html&usg=__JfjGFPE-My5xgZ1H83mralQq0xc=&h=529&w=292&sz=20&hl=en&start=15&zoom=1&itbs=1&tbnid=ZWTp2n_x5tEUnM:&tbnh=132&tbnw=73&prev=/images%3Fq%3Dseven%2Bsegment%2Bdisplay%26hl%3Den%26sa%3DX%26tbs%3Disch:1%26prmd%3Divns

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82C55: Mode 0, Scan Key of Hex

Key board

Row # Col #Data saved

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Debouncing

When a key is depressed, its contact

bounces for a short period of time. This

problem is overcome by either using

hardware de-bouncer or sampling the keysecond time a bit later to assure that the

same key is depressed.


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More than one key of a row is

depressed Techniques to overcome the problem

1. Two key lock out:

During the second scan (after bouncing time is over),if still two keys of a row are found depressed, both

keys will be locked out & neither is accepted bythe P. However if any one of them is releasedafter first scan, the second key still depressed willbe accepted by P.

2. N key roll over:During the second scan (after bouncing time is over),if still more than one key is found depressed, theyare accepted by the P. The key entries areaccepted in the order in which they were pressed.


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Mode 1

Group BGroup A

Port A Upper C Port BLower C


Hand shaking

signals

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Mode 1 Basic functional

Definitions Two Groups (Group A and Group B).

The 8-bit data port can be either input or output.

Pins of port C provide Hand shaking signals.


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82C55: Mode 1 Input Exam.

Keyboard encoder encodes the key-switch into8 bit ASCII code whenever a key is depressed.

DAV (Data Available) is activated on a key

press, strobing the ASCII-coded key code intoPort A.


PC 3

8086

IBF (input bufferfull)

PC 5

INTR

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Example: Mode 1 output

Printer

PB0

PB7

ACKPC2 ACK

8255

PC4 DS

Data Strobe : to tell

the printer to latch the

incoming data.

Generated Externally


8086 INTR

PC 0

1

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Mode 2

Port A C7-C3 Port B


Hand shaking

signalsBidirectional 8 bit

I/O port

Uni directional port with

or without handshaking

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82C55: Mode 2 Bi-directionalOperation

In this mode data is transmitted in both

directions between the 8255 & the peripheral

devices on port A.

Handshaking signals are provided to maintainproper bus flow discipline in a similar manner to

MODE 1.

Port B can be only configured for mode 0 or 1.


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